548 HOLOTHUROIDEA. 



the two vesicles meet on the dorsal side, a mesentery, suspend- 

 ing the alimentary canal and dividing the body cavity longitu- 

 dinally, is often formed. In other parts the partition walls 

 between the two sacks appear to be absorbed. 



The amoeboid cells, which were derived from the invaginated 

 cells, arrange themselves as a layer round all the organs (fig. 

 249). Some of them remain amoeboid, attach themselves to the 

 skin, and form part of the cutis; and in these cells the cal- 

 careous spicula of the larva and adult are formed. Others 

 form the musculature of the larval alimentary tract, while the 

 remainder give rise to the musculature and connective tissue of 

 the adult. 



The development of the vascular system is not known, but the discovery 

 of Kowalevsky, confirmed by Selenka, that from the walls of the water- 

 vascular system corpuscles are developed, identical with those in the blood- 

 vessels, indicates that it probably develops in connection with the water- 

 vascular system. The observations of Hoffmann and Perrier on the commu- 

 nication of the two systems in the Echinoidea point to the same conclusion. 

 Though nothing very definite is known with reference to the development of 

 the nervous system, Metschnikoff suggests that it develops in connection 

 with the thickened bands of epiblast which are formed by a metamorphosis 

 of the ciliated bands of the embryo, and accompany the five radial tubes 

 (vide p. 555). In any case its condition in the adult leaves no doubt of its 

 being a derivative of the epiblast. 



From the above description the following general conclusions 

 may be drawn : 



(1) The blastosphere stage is followed by a gastrula stage. 



(2) The gastrula opening forms the permanent anus, and the 

 mouth is formed by a fresh invagination. 



(3) The mesoblast arises entirely from the invaginated cells, 

 but in two ways : 



(a) As scattered amoeboid cells, which give origin to the 

 muscles and connective tissue (including the cutis) of the body 

 wall and alimentary tract. 



(b} As a portion separated off from the archenteron, 

 which gives rise both to the epithelioid lining of the body cavity, 

 and of the water-vascular system. 



(4) The oesophagus is derived from an invagination of the 

 epiblast, and the remainder of the alimentary canal from the 

 archenteron. 



